It is well known that, for various reasons, humans can develop a condition in which a type of plaque or hard deposit builds up along the walls of the blood vessels, thereby partially blocking the blood flow and causing severe medical conditions. Several different procedures have been developed for dealing with this situation. One such procedure is rotational atherectomy, in which a rotary mechanical system removes relatively hard intravascular deposits from the walls of human arteries by differentially cutting away the inelastic, hardened deposits while sparing the soft, elastic tissue of the inner lining of the human blood vessels. The seminal patent that discloses a device for performing this procedure is U.S. Pat. No. 4,990,134 (Auth) entitled "TRANSLUMINAL MICRODISSECTION DEVICE", the disclosure of which is incorporated herein by reference.
In the commercially available device described in U.S. Pat. No. 4,990,134, known as the Rotablatorl, an ellipsoidal burr coated with tiny diamond chips is rotated at a speed of at least approximately 155,000 revolutions per minute. The burr is connected to a drive motor capable of high speed rotation via a hollow, flexible, helically-wound drive shaft, and is routed through the blood vessel over a narrow guide wire that extends through the central bore of the burr and its drive shaft. When this device is operated, the burr preferentially cuts hard, inelastic material (plaque) while sparing soft, elastic material (tissue) and generates microscopic debris fragments that are sufficiently small in size so as to pass through even the narrowest vascular channels (capillary beds) without clogging them.
This Rotablator.RTM. atherectomy device, as well as any other microdissection device that involves rotational ablation, necessarily generates thermal energy during its rotation. For this reason, as disclosed in U.S. Pat. No. 4,990,134, a biocompatible saline solution is infused through a plastic sheath within which the drive shaft rotates, to cool the sliding interface during operation.
In addition to performing a cooling function, some lubrication is needed to prevent wear caused by rotational friction between the guide wire and the drive shaft or between the drive shaft and the plastic sheath. The major factors that affect wear in this type of rotational contact are load, temperature, surface speed, surface finish, surface hardness, contact area, time, and the type, amount and viscosity of the lubricant.
During extended operation of the device, however, additional lubrication should be provided to sustain the performance of the guide wire, the drive shaft and the sheath. Such a lubricant, if infused through the device from outside the patients' body, must of course, be non-toxic and safe for arterial use. In addition, to be effective in use with the Rotablator.RTM. advancer/guide wire system, the lubricant should be able to withstand shear stresses at 50.degree. C. and should not promote the agglomeration of ablated plaque particles.
Injectable oil-in-water emulsions are currently being used for two clinical applications. The first is for parenteral or intravenous nutrition, as a source of fat calories and essential fatty acids. Examples include Intralipid.RTM., available from Pharmacia and Upjohn and Liposyn.RTM., available from Abbott Laboratories. Emulsions are also being used as a vehicle for poorly water-soluble lipophilic drugs that cannot be injected directly. Examples include Diprivan.RTM., containing the anesthetic drug propofol, and Diazemuls.RTM., containing the drug diazepam.
Lipid emulsions are inherently unstable. No commercially available lipid emulsion is stable following dilution in physiological (0.9% w/v) salt solution. This instability is manifested by formation of large droplets of non-emulsified oil on the surface as well as by a shift in droplet size distribution towards much larger diameters. Such changes often occur within the first hour following dilution in saline and are accelerated by heating or by applying any shear force. The relatively low pH and high ionic strength of saline contributes to this effect.
Commercial lipid emulsions separate into oil and water layers upon thawing after storage at freezing temperatures. For this reason, special care must be taken when shipping in winter through geographic areas having below freezing temperatures. It is preferred that the lubricant be an emulsion which is stable in saline and stable upon freezing with subsequent thawing. The present invention meets these needs and overcomes other deficiencies in the prior art.
What would be desirable is an improved, pharmacologically compatible medical lubricant. What has not been provided is an injectable medical lubricant suitable for lubricating rotating and otherwise moving medical devices.